Anesthetic gas pollution in operating rooms and the health problems of operating room employees have been issues dealt with since 1960, and it is known that health can be impaired by long-term inhalation of anesthetic gas leaking into operating rooms. Anesthetic gas is mixed gas containing nitrous oxide, a volatile anesthetic agent and oxygen, while waste anesthetic gas is the anesthetic gas after respiration by the patient. The composition of waste anesthetic gas is similar to the composition of anesthetic gas, comprising a volatile anesthetic agent, high-concentration nitrous oxide and oxygen. In the United States, the National Institute for Occupational Safety and Health (NIOSH) recommends an environmental exposure level for nitrous oxide (N2O) of no greater than 25 ppm, and for volatile anesthetic agents alone of 2 ppm or no greater than 0.5 ppm in combination with nitrous oxide. It has therefore become mandatory to equip all anesthesia machines with waste anesthetic gas scavenging units, currently allowing operating room environments to meet the aforementioned recommendation.
Nitrous oxide is also used throughout the world for painless childbirth and dental treatment, because of its analgesic and anesthetic effects. Since volatile anesthetic agents are not used for these purposes, the major components of waste anesthetic gas are nitrous oxide, oxygen and carbon dioxide. Likewise, it has become possible for environments in delivery rooms to meet the aforementioned recommendations.
Waste anesthetic gas scavenging units are devices which combine compressed air or the like with waste anesthetic gas from patient exhalation, or eliminate it using a vacuum pump or the like. However, the gas removed from operating rooms, delivery rooms or dental clinics using such waste anesthetic gas scavenging units is currently ejected into the atmosphere without protective measures.
With recent focus on the problem of global warming, the Conference of Parties III (COP3) for Prevention of Global Warming has particularly specified nitrous oxide, together with nitrogen dioxide, methane, freon gas and the like, as a global environmental pollutant which increases global temperatures through a greenhouse effect (a warming effect of approximately 300 times that of carbon dioxide).
In addition, nitrous oxide is also being increasingly used for semiconductor manufacturing processes, heightening the need for measures toward protection of the global environment.
From the viewpoint of global environmental protection, it has become essential to remove or neutralize volatile anesthetic agents and nitrous oxide in waste anesthetic gas when using waste anesthetic gas scavenging units for discharge of waste anesthetic gas, instead of simply discharging it into the air.
Normally, the waste gas discharged from a single operating room, at most medical facilities, is 30-40 L/min including the indoor air accompanying it. However, a variety of waste anesthetic gas scavenging units and discharge methods are used at different medical facilities, and some of these far exceed the treatment capacity of conventional waste anesthetic gas treatment apparatuses. For example, with a discharge line for an entire operating room, including ventilation gas, the treatment volume is 1 m3/min or greater, and the reactor used for treatment at such a flow rate is usually of a type for a small plant. Thus, an increased flow rate requires a large waste anesthetic gas treatment apparatus, resulting in a larger heat exchanger for heating of the large volume of treatment gas to the prescribed temperature and a larger size and volume of its heater, and problems of increased energy consumption and installation space and weight restrictions may arise when the treatment apparatus is installed in a hospital. Particularly when using a system of adsorption removal of volatile anesthetic agents, a large flow of treatment gas results in more rapid breakthrough of the adsorbent, necessitating a much larger unit for implementation and making it difficult to achieve actual practical use.
On the other hand, waste anesthetic gas discharged from delivery rooms and dental treatment clinics consists of nitrous oxide and oxygen, without volatile anesthetic agents, and therefore the units used for its treatment are less complex; however, because of the problems mentioned above, it has not yet been possible to implement units which continuously and efficiently treat the nitrous oxide contained in the waste anesthetic gas at a flow rate of 1 m3/min or greater. For continuous and efficient treatment of nitrous oxide contained in large-volume waste anesthetic gas it is not sufficient merely to increase the size of the treatment apparatus, and limitations also exist on the size of the installation space and on the weight, such that modification of parts are necessary in order to increase energy efficiency and reduce space requirements. Yet such treatment methods and treatment apparatuses are as yet unknown, and therefore in light of the growing awareness of the contribution of nitrous oxide to global warming, a demand exists for development of a treatment method and treatment apparatus capable of continuously treating nitrous oxide-containing gas that is discharged, particularly at large circulation rates, from operating rooms, delivery rooms and dental clinics.
Examples of conventional treatment apparatuses include (1) the high-temperature catalyst unit described in Japanese Unexamined Patent Publication No. 64-511126, (2) the integrated heat exchanger and catalyst reactor described in Japanese Unexamined Patent Publication No. 2004-920, and (3) the contact oxidation unit described in Japanese Unexamined Patent Publication No. 55-56823. The high-temperature catalyst unit proposed in (1) is a reactor having a heat exchanger and a catalyst-filled section in an integrated structure, but because the reactor is built to small specifications, a large circulation flow can result in insufficient heat efficiency of heat transfer at the heater. Also, in the example of an ordinary reactor described in (FIG. 5 of) (1) above, the heater lacks baffles and therefore the heater fails to efficiently utilize heat regardless of the flow rate.
Moreover, in the integrated heat exchanger/catalyst reactor of (2) above, despite the structure of the reactor wherein the heat exchanger and reactor are integrated, the structural characteristics of the reactor allow it to exhibit sufficient treatment capacity for treatment flow rates of up to 1 m3/min, but pressure loss increases with greater flow rates, such that intended flow cannot be achieved in the reactor and adequate treatment cannot be accomplished.
With the contact oxidation unit of (3) above, the heat exchanger and heater are fixed, and therefore any trouble with the heater requires exchange of both the heat exchanger and heater sections, causing a problem in terms of maintenance and cost.
Methods of catalytic decomposition are described, for example, in (4) Japanese Examined Patent Publication No. 61-45486 and Japanese Examined Patent Publication No. 62-27844, wherein nitrous oxide is decomposed with a catalyst. Although these methods can decompose high concentrations of nitrous oxide, the nitrogen oxides nitrogen monoxide (NO) and nitrogen dioxide (NO2) (hereinafter referred to as NOx) are produced at 5-32 ppm, in some cases leading to the problem of NOx production exceeding 3 ppm as the permissible concentration for NO2 (TWA: time-weighted average). In addition, the method proposed in (4) above requires a contact time of 0.2 second or longer, i.e. a space velocity (SV) of no greater than 18,000 Hr−1, and therefore the treatment volume is limited. A larger SV shortens the contact time, resulting in problems such as reduced reaction efficiency.